Heat generating systems embodying furnaces for combusting fossil fuels have long been utilized to generate controlled heat, with the objective of doing useful work. The work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators for industrial or marine applications or for driving turbines that produce electric power. Modern water-tube furnaces for steam generation can be of various types, including but not limited to, fluidized-bed boilers. While there are various types of fluidized-bed boilers, generally speaking, all operate on essentially the same principle wherein a gas, commonly in the form of air, is injected to fluidize solids prior to the combustion of fuel within the fluidized-bed boiler.
In accordance with the mode of operation of circulating fluidized-bed (CFB) type boilers typically a gas, e.g., by way of exemplification, air, is made to flow through a bed of solid particles in order to thereby produce forces that are capable of effecting therewith a separation of the solid particles from one another. To this end, as the flow of gas is increased, a point is thus reached at which the forces produced by such gas, which are applied to the solid particles, are just sufficient enough to cause separation of the solid particles from one another. When this point is reached, the bed of the CFB boiler becomes fluidized such that the cushion of gas that is present between the solid particles allows the solid particles to move freely, thereby giving the bed of solid particles of the CFB boiler the characteristics of a liquid. The bulk density of the bed of solid particles of the CFB boiler is relatively high at the point in time when the solid particles that comprise the bed of the CFB boiler become fluidized, but the bulk density thereof will decrease as the bed of solid particles is made to flow upward through the CFB boiler where the solid particles are combusted to generate heat.
The solid particles, which are made to circulate within the CFB boiler, typically include solid fuel particles, such as crushed or pulverized coal or other solid fuel, and sorbent particles, such as crushed or pulverized limestone, dolomite or other alkaline earth material. Combustion of the solid fuel particles within the CFB boiler results in the production of flue gas and ash. During such combustion of the solid fuel particles, the carbon in the solid fuel particles becomes oxidized thereby resulting in the generation of carbon dioxide (CO2). Nitrogen is also oxidized during such combustion of the solid fuel particles thereby resulting in the generation of nitrogen oxide (NOx). Additionally, during such combustion of the solid fuel particles sulfur is oxidized to form sodium dioxide (SO2). Such CO2, NOx, SO2 as well as the other gases that are generated during the combustion of the solid fuel particles within the CFB boiler collectively together comprise what is commonly referred to as the flue gas. The ash produced during the combustion of the solid fuel particles within the CFB boiler, generally speaking, consists primarily of unburned solids in the form of inert material and sorbent particles. Such ash, or at least some portion thereof, is sometimes also referred to as particulate matter. The ash, which is produced from the combustion of the solid fuel particles within the CFB boiler, becomes entrained in and is carried in an upwardly flow path within the CFB boiler by the hot flue gas that is produced during such combustion of the solid fuel particles. This ash thereafter is exhausted from the CFB boiler along with the hot flue gas. During this upwardly flow of the hot flue gas within the CFB boiler, the SO2 in this hot flue gas is designed to be absorbed by the particles of sorbent to which reference has been made hereinbefore.
In accordance with the present invention an air pollution control (APC) subsystem of conventional construction is preferably employed to remove various so-called pollutants, including, by way of exemplification, CO2, NOx, SO2 and particulate matter, from the flue gas that is generated from the aforementioned types of heat generating systems. To this end, such flue gas that is exhausted from the CFB boiler is directed to the various components of an APC subsystem of conventional construction before reaching the stack and before being exhausted from the stack into the atmosphere. Each of the components of such an APC subsystem can be considered to be a system in such a component's own right. For instance, by way of exemplification and not limitation in this regard, such flue gas may be subjected to being processed through the use of a cyclone separator and/or an electrostatic precipitator in order to thereby remove the particulate matter from such flue gas, viand may be subjected to being processed through the use of a selective catalytic reduction (SCR) system in order to thereby remove the NOx from such flue gas, and may be subjected to being processed through the use of a SO2 scrubber system in order to thereby remove the SO2 from such flue gas, and may be subjected to being processed through the use of a CO2 scrubber system in order to thereby remove the CO2 from such flue gas.
However, there are other ways that are also known to be operative for purposes of effecting therewith the reduction of emissions from a flue gas. For example, it is known in this regard that CO2 and NOx emissions can be reduced by employing oxygen in the combustion process. More specifically, to this end in U.S. Pat. No. 6,505,567, which was issued on Jan. 14, 2003 that is entitled “Oxygen Fired Circulating Fluidized Bed Steam Generator”, and which is assigned to the same assignee as that to which the present application is assigned, there is described and illustrated a CFB boiler steam generating system that employs oxygen, rather than air, to effect therewith the fluidization of the fuel in the CFB boiler. The CFB boiler steam generating system that is described and illustrated in U.S. Pat. No. 6,505,567 facilitates the use of CO2 both as a desired end product and as a means of support for the combustion process. By virtue of this reference thereto the disclosure of U.S. Pat. No. 6,505,567 is hereby incorporated herein in its entirety.
In accordance with the technique that is described and illustrated in U.S. Pat. No. 6,505,567, a substantially pure oxygen feed stream is introduced into the CFB boiler of the steam generating system. Fuel is then combusted in the presence of this substantially pure oxygen feed stream as well as in the presence of a recirculated flue gas stream in order to thereby produce a flue gas in which CO2 and water vapor are the flue gas' two largest constituent elements by volume, and a flue gas, which is substantially free of NOx. Such flue gas is designed to be made to flow through an oxygen feed stream pre-heater, the latter being operative to transfer heat from such flue gas to the oxygen feed stream. With further reference thereto, such flue gas is then separated into an end product portion and a recycling portion. Such end product portion of the flue gas is thereafter subjected to being cooled and compressed in order to thereby yield CO2 in a liquid phase. On the other hand, the recycling portion of the flue gas is thereafter directed back to the CFB boiler of the steam generating system for purposes of thereby contributing to the combustion process therein.
The technique that is disclosed and illustrated in U.S. Pat. No. 6,505,567 provides one with the flexibility to be able to use as a desirable end product the CO2 that is produced therefrom as well as to be able to support the combustion process therewith. The production of liquid CO2 also functions to improve the heat output of the CFB boiler steam generating system. However, while the technique that is disclosed and illustrated in U.S. Pat. No. 6,505,567 can be used for purposes of significantly reducing the amount of CO2 emissions, there nevertheless remains a reluctance by some in many quarters to provide additional coal fired steam generating system capacity because of their concerns that there may be enacted future governmental regulations regarding CO2 emissions and the costs that may be required to be incurred in order to thereby meet such regulations. In this regard, studies have shown, by way of exemplification, that the investment required in terms of the costs to retrofit conventionally constructed coal fired CFB boiler steam generating systems for CO2 capture typically would be in the range of $1000 to $1600 per kilowatt (kW). Such studies have also shown that the energy penalty for effecting such CO2 capture in the case of coal fired CFB boiler steam generating systems typically would range from 25% to 40%. With further reference thereto, particularly in the case of retrofit situations, the site itself of such coal fired CFB boiler steam generating systems may be insufficient to accommodate a CFB boiler steam generating system of the type that is described and illustrated in U.S. Pat. No. 6,505,567.
Thus, while it is recognized that a need exists for providing more steam generating system capacity in order to thereby produce, for example, additional electrical power, it is also recognized that coal fired CFB boiler steam generating systems are an efficient means for effecting therewith the generation of such steam. Nevertheless, in view of the ongoing debate over global warming, as well as in view of the increasing attention being directed to CO2 emissions from the burning of fossil fuels such as coal, the cost in particular of capturing CO2 in terms of both the capital expense required therefor and the reduced energy production provided thereby, undoubtedly have resulted in the delaying of at least some installations, which could otherwise have provided the increase in capacity that is required, and thereby thus increase the availability of power that both the nation and the world require.
Accordingly, a need has been found to exist in the prior art for a new and improved technique operative for purposes of effecting therewith the capture of the CO2 that is generated by fossil fuel fired steam or other heat generating systems during the operation thereof.